MRI Response to Pre-operative Stereotactic Ablative Body Radiotherapy (SABR) in Early Stage ER/PR+ HER2- Breast Cancer correlates with Surgical Pathology Tumor Bed Cellularity

• Published in: Clinical breast cancer 2022-02, Vol.22 (2), p.e214-e223
• Main Authors: Weinfurtner, R. Jared, Raghunand, Natarajan, Stringfield, Olya, Abdalah, Mahmoud, Niell, Bethany L., Ataya, Dana, Williams, Angela, Mooney, Blaise, Rosa, Marilin, Lee, Marie C., Khakpour, Nazanin, Laronga, Christine, Czerniecki, Brian, Diaz, Roberto, Ahmed, Kamran, Washington, Iman, Montejo, Michael
• Format: Article
• Language: English
• Subjects: Adult; Aged; Breast Cancer; Breast MRI; Breast Neoplasms - metabolism; Breast Neoplasms - pathology; Breast Neoplasms - radiotherapy; Female; Follow-Up Studies; Humans; Magnetic Resonance Imaging; Middle Aged; Monitoring; Neoadjuvant Radiation Therapy; Pathology, Surgical - methods; Radiotherapy, Intensity-Modulated - methods; Receptor, ErbB-2 - metabolism; Receptors, Estrogen - metabolism; Retrospective Studies; Treatment Response
• ISSN: 1526-8209; 1938-0666
• DOI: 10.1016/j.clbc.2021.06.016

•Key Finding: In early stage ER/PR+ HER2- breast cancers treated with neoadjuvant radiation therapy, the percent volume of tumor remaining on post-SABR MRI compared to baseline correlates with surgical pathology percent tumor bed cellularity.•Importance: This finding allows pre-surgical assessment of neoadjuvant radiotherapy response in these ER/PR+ HER2- tumors for which pathologic complete response is rare. This study evaluates breast MRI response of ER/PR+ HER2- breast tumors to pre-operative SABR with pathologic response correlation. Women enrolled in a phase 2 single institution trial of SABR for ER/PR+ HER2- breast cancer were retrospectively evaluated for radiologic-pathologic correlation of tumor response. These patients underwent baseline breast MRI, SABR (28.5 Gy in 3 fractions), follow-up MRI 5 to 6 weeks post-SABR, and lumpectomy. Tumor size and BI-RADS descriptors on pre and post-SABR breast MRIs were compared to determine correlation with surgical specimen % tumor cellularity (%TC). Reported MRI tumor dimensions were used to calculate percent cubic volume remaining (%VR). Partial MRI response was defined as a BI-RADs descriptor change or %VR ≤ 70%, while partial pathologic response (pPR) was defined as %TC ≤ 70%. Nineteen patients completed the trial, and %TC ranged 10% to 80%. For BI-RADS descriptor analysis, 12 of 19 (63%) showed change in lesion or kinetic enhancement descriptors post-SABR. This was associated with lower %TC (29% vs. 47%, P = .042). BI-RADS descriptor change analysis also demonstrated high PPV (100%) and specificity (100%) for predicting pPR to treatment (sensitivity 71%, accuracy 74%), but low NPV (29%). MRI %VR demonstrated strong linear correlation with %TC (R = 0.70, P < .001, Pearson's Correlation) and high accuracy (89%) for predicting pPR (sensitivity 88%, specificity 100%, PPV 100%, and NPV 50%). Evaluating breast cancer response on MRI using %VR after pre-operative SABR treatment can help identify patients benefiting the most from neoadjuvant radiation treatment of their ER/PR+ HER2- tumors, a group in which pCR to neoadjuvant therapy is rare. This study compares MRI response to pathologic response in 19 pre-operative SABR-treated ER/PR+ HER2- breast cancers. Analysis of tumor % volume remaining on MRI post-SABR compared to baseline demonstrates linear correlation with pathologic % tumor cellularity. This helps identify patients benefiting from neoadjuvant radiation treatment, a group in which complete pathologic